[modules] Make sure we always include the contents of private headers when

building a module. Prior to this change, the private header's content would
only be included if the header were included by another header in the same
module. If not (if the private header is only used by the .cc files of the
module, or is included from outside the module via -Wno-private-header),
a #include of that file would be silently ignored.

llvm-svn: 257222